A reinvestigation of lagoon specific local marine reservoir effects in Tongatapu, the Kingdom of Tonga, over the last 3000 years
Dr Naoto Fukuyo1, Professor Geoffrey Clark2, Professor Yusuke Yokoyama1,3
1Atmosphere And Ocean Research Institute, The University Of Tokyo, , Japan, 2Archaeology and Natural History, College of Asia and the Pacific, The Australian National University, Canberra, Australia, 3Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, , Japan
Lagoon-specific local marine reservoir effects (ΔRlagoon) were defined in Fukuyo et al. (2020) as the difference between the ¹⁴C activity of the open ocean and lagoon-specific values from the same time period to estimate the 14C activity change in the lagoon’s freshwater proportion. This study also reported that ΔRlagoon values from fossil bivalves of Gafrarium tumidum excavated from archaeological sites in the Fanga ‘Uta Lagoon, located on the north coast of Tongatapu Island, the Kingdom of Tonga. ΔRlagoon values in the lagoon increased linearly from 220 to 370 years between ~2.6 and 0.4 ka. These values suggested that the main cause of ΔRlagoon value increase was the closure of the Fanga ‘Uta Lagoon due to relative sea-level (RSL) changes, which caused an influx of radiocarbon depleted water sourced from on-land Tongatapu Pleistocene limestone. However, ΔRlagoon values in Fukuyo et al. (2020) had large errors, and the calibration was conducted before the publication of the latest and more precise calibration data sets ShCal20 and Marine20. Here, we present new 15 ΔRlagoon values from G. tumidum fossils, combine these with the published values from Fukuyo et al. (2020), and recalculate ΔRlagoon using Marine20. Although the newΔRlagoon values were similar between 0.4 ka and 1.2 ka (160 to 170 years), the values increased from 2.6 ka to 1.2 ka (100 to 160 years). Fukuyo et al. (2020) reconstructed RSL changes between 2.6 and 0.4 ka using glacio-hydro-isostatic adjustment (GIA) modeling. The results of GIA modeling suggest RSL decreased sharply after 2.25 ka. This timing aligns with anΔRlagoon increase of approximately 50 years between 1.2 ka and 2.6 ka. In contrast, the absence of RSL change between 0.4 ka and 1.2 ka suggested freshwater influence in the lagoon was similar to the environment, and thus, ΔRlagoon was small. These results were consistent with paleotopographical comparisons reported in Fukuyo et al. (2020) and an archaeological study reported by Clark et al. (2015).
Clark G, Grono E, Ussher E, Reepmeyer C. Early settlement and subsistence on Tongatapu, Kingdom of Tonga: Insights from a 2700–2650calBP midden deposit. Journal of Archaeological Science: Reports. 2015;3: 513–524. doi:10.1016/j.jasrep.2015.08.005
Fukuyo N, Clark G, Purcell A, Parton P, Yokoyama Y. Holocene sea level reconstruction using lagoon specific local marine reservoir effect and geophysical modeling in Tongatapu, Kingdom of Tonga. Quat Sci Rev. 2020;244: 106464. doi:10.1016/j.quascirev.2020.106464
Dr. Naoto Fukuyo is a postdoctoral researcher at the AORI, The University of Tokyo, working on unveiling the relationships between environments and human activity. His current research focuses on paleoenvironmental and paleo sea-level reconstruction using radiocarbon dating and speleothem magnetism in the Pacific.